1. Technical Field
The present invention relates to a protective layer for piston rings in internal combustion machines, essentially consisting of chromium carbides, tungsten carbide, chromium and nickel.
2. Related Art
The contact surfaces of piston rings in internal combustion engines are subject to wear during their use. To minimize the wear the bearing surfaces of piston rings are supplied with a protective layer. Depending on the production method utilized, it is part of background art to produce these layers by means of a high-speed flame spraying method. In this procedure the coating material, which is present as powder, is fused by means of an oxygen/fuel spray gun and sprayed onto the piston ring. EP 0 960 954A2 discloses a corresponding powder for generation of these protective layers against wear. This powder contains nickel, chromium and carbon, whereby the chromium can be present as chromium-carbide and nickel-chromium alloy. The essay “The Application of Cermet Coating on Piston Ring by HVOF” by H. Fukutome from 1995, of the Japanese piston ring manufacturer Teikoku Piston Ring, also describes the use of chromium carbides and nickel-chromium alloys for generating protective layers against wear by means of high-speed flame spraying. The alloy components used in both publications form a nickel-chromium matrix, in which depending on the alloying contribution chromium-carbides are embedded. The drawback to these coatings is that, due to their hardness and brittleness they are subject to cracks, whereby the susceptibility to cracking can even be the determining factor for the service life of the piston rings. This susceptibility to cracking results from the great carbide diameters, which, when conditioned by stress leads to carbide fractures and thus to wear on the rings. In particular in the plasma powders the carbides are present in an already decomposed form, so that the matrix embrittles and the carbide loses hardness through transformation of Cr3C2 to Cr7C3 or even to Cr23C6. To oppose this drawback, in DE 197 20 627 A1 20 to 80 Vol-% of molybdenum is mixed into the spray powder. Molybdenum possesses a relatively high viscosity and can thus stop the crack growth. The patent application discloses preferred coatings of sintered chromium-carbide and nickel chromium powders with up to 100% weight molybdenum. By means of introducing the molybdenum into the powder, however, in the resulting coating phases made of molybdenum come into being, which are roughly the size of the initial powder and as a rule have a diameter of 5 to 50 μm. The relatively low resistance to abrasion of the molybdenum has a negative effect, the molybdenum phases are preferably worn out and consequently reduce the protective layer's resistance to wear.
Along with the chromium carbides, tungsten carbides are also embedded into the matrix of the protective layer against wear. The European patent publication EP 0 512 805 B1 describes the formation of a surface protection with chromium and tungsten carbides, whereby the embedded tungsten-chromium-carbides have a particle size in the range of 25–100 μm. Tungsten carbides are harder than chromium carbides and possess a very high resistance to wear and pressure. The extraordinarily hard tungsten carbides, however, show a significant disadvantage in the processing of the produced surface. The surface can no longer be finished with conventional grinding wheels, processing is only possible with very high-quality and at the same time expensive grinding wheels.